Despite decades of feminist consciousness-raising, sexist language still exists in our culture. Gender-specific titles and pronouns can subtly influence sexism as well as our thoughts and expectations about gender roles and appropriate occupations and goals for the sexes.
Sexist language is language that excludes either men or women when discussing a topic that is applicable to both sexes. This includes using the word man to refer to humanity, and using titles like Congressman and fireman. Another common error that shows gender bias is assuming that the subject of all sentences is male. For instance, the statement "Each student chose his own topic for his term paper," leads the reader to assume that all the students in the class were male, despite the probability that half of them were female.
Underlying sexist language is gender bias, which can occur consciously or unconsciously. When unconscious, the gender bias in language can be considered to be the product of society: other people use sexist language, and repetition normalizes it until the speaker unconsciously produces his or her own sexist language where men are the norm and women the "other".
Sexist language encourages discrimination and can discourage people from pursuing their dreams. If engineers are always spoken of as male, a girl who aspires to be an engineer may feel that she has no hope, since "all" engineers are men.
Sexist language also offends people when they find themselves excluded. This is not an issue that violates your right to free speech; you are free to use offensive language, and also free to decide that you do not agree with the aforementioned definition of what constitutes sexist language. However, if you are using language that is offensive to half of your audience, you will not get your message across. People will not be receptive to your arguments if they are aggrieved by your use of exclusively masculine pronouns.
Remember that the goal is not to avoid referring to individual people as male or female; the goal is to be inclusionary when speaking in hypothetical statements or of mixed-gender groups.
Note: Do not take gender-inclusivity to extremes; each specific individual has a gender, and can be referred to in that way. Likewise, some biological facts apply only to women or only to men. If you are writing a paper on giving birth, you should not refer to your subject as "a pregnant woman or man".
The use of sexist language undermines the American goal of a non-discriminatory, inclusive society. There are simple ways to avoid using language that can offend and marginalize half the people on earth, so writers should take care to use gender-inclusive language.